1. Field of Invention
The present invention relates to the field of digital communications. More specifically, the present invention provides for a novel initialization system and method for discrete multitone modems.
2. Background of Invention
The demand for high-speed data rate services has been increasing very rapidly. One way to meet this demand is to expand the existing infrastructure by connecting all users of bandwidth-consuming applications to fiber optic networks. However, this solution is not financially viable. An attractive alternative to fiber optics involves the transfer of information at high-speed data rates over ordinary telephone networks using Orthogonal Frequency Division Multiplexing (OFDM). When applied in a wireless environment, it is generally referred to as OFDM, while in a wired environment the term Discrete Multitone (DMT) is more appropriate.
The history of OFDM dates back to 1966, when Robert W. Chang published his paper on the synthesis of bandlimited signals for multichannel transmission without interchannel interference (ICI) and intersymbol interference (ISI). Chang was awarded a patent for his work in 1970. In 1971, S. B. Weistein and P. M. Ebert contributed to OFDM with the use of Discrete Fourier Transform (DFT) to perform baseband modulation and demodulation, removing the need to use banks of subcarrier oscillators. But, Weistein and Ebert's system could not guarantee orthogonality. In 1980, A. Peled and A. Ruiz solved the orthogonality problem with the introduction of the cyclic prefix (CP). OFDM is currently being used as the European Digital Audio Broadcasting (DAB) standard and under the name DMT, has been chosen as the modulation scheme for ADSL, a technique for transmission over twisted pair telephone lines standard and is a major contender in the ongoing VDSL standard for high-speed transmission on the existing telephone network.
The principles of multicarrier modulation are described more completely in “Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come”, by John A. C. Bingham, IEEE Communications Magazine, Vol. 28, No. 5, pages 5–14, May 1990. As is known in a modem system utilizing multicarrier modulation such as DMT, the bits of input data for transmission within each block or symbol period are allocated to sub-carriers.
At the receiver DMT signals are subject to synchronization errors due to oscillator impairments and sample clock differences. The effective implementation of DMT requires that the received signal be recovered with a near perfect synchronization. Two types of synchronizer structures are well known in the art. Data-aided synchronizers use the receiver's decisions or a training sequence to compute the timing offsets. Non-data-aided synchronizers operate independent of the transmitted information sequence. For the NDA approach, the timing estimates maximize the NDA likelihood function, which is obtained by averaging the likelihood function over the random information variables. A thorough explanation of DMT synchronization can be found in, “Synchronization with DMT Modulation”, Thierry Pollet and Miguel Peeters, IEEE Communications Magazine, April 1999, and in, “ML Estimation of Time and Frequency Offset in OFDM Systems”, Jan-Jaap van de Beek, IEEE Transactions on Signal Processing, Vol. 45, No. 7, July 1997, both herein incorporated by reference.
It is also necessary for the receiver to perform channel estimation, which allows for compensation of the effects of the transmission medium (ex. telephone line channel). Prior art systems for channel estimation are well known in the art. Channel estimation in OFDM systems is usually performed with the aid known pilot symbols. A detailed explanation of channel estimation and pilot tones can be found in, “Pilot Tone Selection for Channel Estimation in a Mobile OFDM System”, Rohit Negi and John Cioffi, IEEE Transactions on Consumer Electronics, Vol. 44, No. 3, August 1998, incorporated herein by reference.
While many techniques for synchronization and channel estimation are known in the art, the prior art systems require complete synchronization prior to channel estimation. A system that efficiently combines synchronization and channel estimation will serve to significantly reduce the start-up time of any communication systems.